Seismic measurement systems typically have some form of energy source to transmit an impulse of energy into the earth which, when reflected from various substructures in the earth, can be measured at the earth's surface with the use of sensors. This information is then collected and correlated with the input energy to provide a translation of the structural arrangement beneath the earth's surface.
One type of system for performing the seismic measurement utilizes a vibrating base plate that is vibrated at a relatively low frequency of between 5-100 Hz. Normally, the frequency of vibration is swept over this range, which vibration energy is transmitted to the earth through the vibration plate. Sensors are arranged in the earth along a line and spaced apart approximately 25-50 meters. The vibration device is moved along this line of sensors, placed upon the ground at predetermined points and the vibration energy transmitted to the earth. Along the line of sensors or elsewhere, a recording station is located which receives the output from each of the sensors. This data is collected and input to one input of a data correlator. The other input of the correlator is a model of what the output energy of the vibration device is expected to be.
Typically, each of the vibration devices has a reference oscillator located on board which is triggered to sweep from between a low frequency and a high frequency in one or several sweeps. If multiple vibration devices are utilized, they are all triggered at the same time. However, for the data correlator to work, optimally it is desirable for the model swept frequency to have the same amplitude and phase relationship within all vibration type devices in order to provide accurate results.
In present vibrator systems, each of the vibration devices is calibrated against a reference oscillator at the recording station, which reference oscillator is used to input the model input signal to the data correlator. However, there exists a possibility that there can be some deviation of phase and/or amplitude between the vibration devices themselves as well as deviation from the reference at the recording station. To insure the integrity of the data, systems in the past have calibrated this frequency and phase relationship prior to initiating a measurement sequence and then have checked it again at a later time. If the deviation is minimum, this provides some confidence in the integrity of the measurement. However, this approach does not take into account any of the practical limitations of the vibration devices in transmitting the swept frequency vibration to the earth. Any variations will result in noise and, thus, inaccurate data.